Not just one kind of overlapping, either. Locomotive liveries overlapped between 'periods', but it doesn't stop there. Those changes happened at different times and in different ways to livery changes on, say, coaching stock. And then there's the changes in wagons, signals, track, stations, buildings, staff clothing, etc, etc. There were, of course, many changes in how those things were designed and used. And that's without mentioning changes in the rest of the country through which the railway runs!
The notion of 'periods' seems a bit arbitrary when you get down to the detail. Even if we concentrate on one narrow category, say locomotives (which seems to be a favourite), do we measure time by changes in liveries? Or in rebuilds? Or in the introduction of new kinds of safety equipment and other fittings? And so on ...
What one person sees as an important change is hardly noticed by others. But all of these changes matter, because they're all part of the overall picture. The best common denominators are time and place.